A process and device for the manufacture of flat glass sheet with improved characteristics

Abstract: In manufacture of a flat glass sheet using the float process (1), before annealing of glass carried on rollers (3), the glass is conveyed by means (2) of a fluid bed formed by a mixture of gasses, in order to cool the glass to approximately 600°C before it reaches the annealing lehr (Figure 1).

Description: The present invention relates to a process and a device for the manufacture of flat glass sheets with improved optical and mechanical characteristics. In particular, it relates to an improvement in the transfer operation of the glass sheet strip from the molten metal tin bath to a roller annealing lehr within the manufacturing process of flat glass sheets using the "float" method, known by the state of the art; furthermore, it also relates to a device for carrying out said improvement. It is a known fact that flat glass sheets are manufactured in plants providing a first stage in which the raw materials are melted and a subsequent stage in which the glass sheet is formed by floating and sliding the molten material on a tin bath. These plants, known by the state of the art as float plants, provide an annealing lehr downstream of the of molten tin bath, in order to provide the glass sheet strip with the required mechanical characteristics. The PCT patent application No. 92/16466 filed by the GLASSTECH company describes a float plant equipped with an annealing lehr in which the glass sheet strip travels while being supported on fluid bed. US-A-3,506,422 describes a method and apparatus for forming a flat glass on a fluid support bed in which a ribbon of glass is continously lifted from a supporting molten metal bath onto a gaseous cushion on which the ribbon is conveyed and conditioned in its temperature before reaching an annealing lehr so as to avoid damage to its soft undersurface. The aim of this device is to provide an annealing process in which the glass sheet strip does not come into contact with the rollers, as is the case in the conventional method, in order to improve the optical and mechanical characteristics of the glass surface itself, being in this stage still at a high temperature, can undergo damage when in contact with rollers. The main inconvenience with the above described device consists in the fact that it is necessary to carry out expensive modifications to the existing systems, as the integral replacement of the annealing lehr, which is several dozen meters long becomes necessary. Another inconvenience lies in the fact that when manufacturing a new float plant, the realization of an annealing lehr having an entirely fluid bed requires high investments; furthermore the process control is more difficult. The aim of the present invention is to overcome the above mentioned problems. It has been surprisingly found that an identical improvement in the optical and mechanical characteristics of the glass sheet strip surface is obtained by placing between the tin bath and the traditional annealing lehr entrance, equipped with conveying rollers, a device that generates a fluid bed which serves to cool the strip of glass sheet strip and to support it adequately as it travels towards the next roller annealing lehr. An object of the present invention is therefore a process for manufacture of flat glass sheets with improved optical and mechanical properties, including melting a raw material in a furnace, forming a glass sheet strip in a bath of molten metal tin, conveying said glass sheet strip from said molten tin bath in a direction of travel by on annealing lehr provided with rollers, and pre-cooling said glass sheet strip while supported by a fluid bed at a stage of travel between said bath of molten tin and said roller annealing lehr, characterized by the steps of forming said fluid bed by two different mixtures of gases in succession to the travel direction of the glass strip, and emitting said mixtures of gas by a first module of said fluid bed and by a second module of said fluid bed, said first and second modules being adjacent in said succession but not in fluid communication with each other. This process reinforces the surfaces of the glass, preventing it from being damaged by next contact with the rollers, and allowing the next annealing operation to be performed in a conventional roller lehr without the danger of damaging the surface of the glass and the reduction of optical features. The fluid bed is formed by a suitable mixture of gasses, at a controlled temperature and flow rate, in order to allow the glass sheet strip to reach the entrance to the roller annealing lehr at a chosen temperature, preferably not exceeding approximately 570°C (+/- 10°C), whatever the temperature of the glass upon exiting the tin bath. It is preferred that the temperature of the glass upon entering the device according to the present invention be between 650°C and 600°C, according to the thickness of the glass and the speed at which it is conveyed, so that the thermal head from 600 - 650°C upon entering the pre-cooling stage and 570°C upon leaving it can be obtained and controlled through the temperature and flow rate of the gasses in the fluid bed. The annealing of the glass sheet strip by the gradual lowering of the temperature from approximately 570°C to approximately 495°C is then performed as usual in the annealing lehr equipped with rollers. Preferably, the mixture of gasses emitted by the first module is a mixture of hydrogen and nitrogen, in which the hydrogen is included in an amount of between 4 and 8% of the nitrogen; and the second module provides a mixture of air and SO2, in which the SO2 is present in a percentage ranging between 10 and 12% with respect to the air. Advantageously the process can also be used to deposit thin layers on one or both the surfaces of the glass. In this case the mixture of gasses emitted by the second module is a mixture of air and organic metal elements (for example Ti, Fe, Sn, Sb, Co, Zn, Zr, Si, etc.) suitably prepared for chemical vapour phase deposition. Furthermore the process can also be advantageously used to improve cohesion between the substrate and the thin layer, by providing for emission of suitably chosen mixed gasses (for example in this case a mixture of fluorine and sulphur dioxide will be used in the first part, and a mixture of air and organic metal elements in the second part). The invention also provides a device according to claim 7. Preferably the nozzles are divided into groups whose flow rate can be independently adjusted so as to adapt to the different sizes and flatness of the strip of glass. In other words, the process and the device described above and forming the object of the present invention allow a number of technical problems to be solved simultaneously: improvement of the optical and mechanical characteristics of the glass surface, thin layers deposition, cohesion improvement between the surface of the substrate and the thin layers deposited thereon, while also giving the advantage of notably reducing installation costs, as it is only necessary to install a single, multi-purpose device. All the above is possible thanks to the fact that the fluid bed device in question is installed in the area between the tin bath and the annealing lehr. Additional characteristics and details of the invention will be provided with the aid of the following drawings, given merely as non-limiting examples, in which a preferred embodiment of the device realized according to the present invention is shown, and in which: figure 1 is a vertical section view of the device in accordance with the present invention; figure 2 is a vertical cross-section view of the device in accordance with the present invention; and figure 3 is a vertical section view of the blowing nozzle. With reference to figures 1 to 3 and according to the present invention, the fluid bed device 2 in conformity with the invention is positioned between the tin bath 1 and the roller annealing lehr 3, where the travel direction of the glass sheet strip is indicated by the arrow A. The fluid bed device 2, according to figure 1, is made up of two sections: the section closest to the tin bath 1 is indicated with 2a, and has a length not exceeding 500 millimeters; the following section 2b has a length not exceeding 1000 millimeters. The fluid bed device is divided into two sections so that it can operate with different mixtures, and in addition, in case of malfunction in one of the two sections, to enable the faulty section to be removed, while continuing to work with the other one. The existence of two modules, one above S and one below I compared to the travelling level of the glass strip, makes it possible to intervene using suitable mixtures, at suitable temperatures, on both sides of the glass strip V. Each module has a tank 4 for re-circulation of the mixture and a tank 5 for preparation of the air-gas mixture and distribution of the mixture through the conduits 8. The re-circulation fans 6 cause the mixture to flow from the re-circulation tank 4 to the preparation tank 5. Each conduit 8 has an adjustment valve 7 to adjust the flow rate of the gasses in the fluid bed. A temperature adjuster 9 is positioned between the re-circulation tank 4 and the distribution tank 5 to maintain the temperature of the fluid at the desired level, in order to control the thermal gradient of the glass sheet strip during its travel from the tin bath to the annealing lehr 3. A fan 10, connected by conduit 13 to a valve 11 and a tank 5, maintains the flow rate of the fluid at the desired level. Feeder pipes 12 supply the gas necessary to form the mixture for the fluid bed. The tanks 5 are supported by jacks 14 resting on plates 15 which in turn rest on jacks 16, so that the group of tanks 4 and 5, which are integral with each other, can be lowered or raised with respect to the level of glass sheet strip, according to need. As can be seen from figure 1, the fluid bed device seen in cross-section and the fluid bed generated thereby are inclined when compared to the horizontal by an amount preferably comprised between 1.2° and 5°. With reference to figure 2, the group of tanks 4 and 5, which are integral with each other, is preferably divided into modules having different widths, which decrease from the center of the glass sheet strip V outward, in order to allow greater flexibility of adjustment in the peripheral areas of the glass strip V, where temperature distribution is less even when compared with the central areas. Cooling fans 17 are used to maintain the fans 6 re-cycling the fluid bed at a constant temperature. The sides of the fluid bed device are closed by using moving doors 18 provided with heating elements 19 to compensate any heat loss at the edges. The nozzles generating the fluid bed are shaped so that they allow the glass strip V to be supported on one side, and optional chemical vapour phase deposition of metal oxides to form thin layers on the other side. In particular, distribution of the mixture takes place by means of the nozzles 20. These nozzles are arranged in the module I below and the module S above the glass sheet strip, so that deposition can take place on the upper or lower surface of the strip of glass. Each of these nozzles has a coating 21 of refractory material, in which the shape of the slot 22 for emission of the gaseous mixture is formed. Said slot 22 is inclined in the same direction as the direction of movement of the glass sheet strip, in order to reduce gas turbulence to a minimum and thus allow the best possible conditions for chemical deposition of thin layers. The block of refractory material 21 is in turn hinged around hinge 23 and undergoes the thrust of a piston 24. This piston, according to requirements, alters the inclination of the refractory material block, and thus of the slot 22, with respect to the horizontal.

Claims: A process for manufacture of flat glass sheets with improved optical and mechanical properties, including melting raw materials in a furnace, forming a glass sheet strip on a bath of molten tin (1), conveying said glass sheet strip (V) from said molten tin bath in a direction of travel (A) to an annealing lehr (3) provided with rollers, and pre-cooling said glass sheet strip while supported by a fluid bed at a stage of travel between said bath of molten tin and said roller annealing lehr, thereby controllably lowering the glass temperature itself by the cooling and supporting action of mixtures of gasses, characterized by the steps of forming said fluid bed by two different mixtures of gases emitted by a first module (2a) of said fluid bed and by a second module (2b) of said fluid bed, said first and second modules being located adjacent to each other in the direction of travel but not in fluid communication with each other. A process according to claim 1, in which the two different mixtures of gasses are each emitted below and above the glass sheet strip (V). A process according to claim 1 or claim 2, characterized by the fact that during said pre-cooling stage the temperature of the glass strip (V) is reduced from a level of between 650 and 600°C to a level of between 580 and 560°C over a distance of approximately 1500 mm. A process according to any of claims 1 to 3, characterized by the fact that the first mixture of gasses emitted by the first module (2a) is made up of nitrogen and of an amount of between 4 and 8% hydrogen; the second mixture of gasses emitted by the second module (2b) is made up of air and of an amount of between 10 and 12% sulphur dioxide. A process according to any of claims 1 to 3, characterized by the fact that the first mixture of gasses emitted by the first module (2a) is made up of nitrogen and of an amount of between 4 and 8% hydrogen, and the second mixture of gasses emitted by the second module (2b) is made up of air and organic metal elements for chemical vapour phase deposition of thin layers on one or both surfaces of the glass strip (V). A process according to any of claims 1 to 3, characterized by the fact that the first mixture of gasses emitted by the first module (2a) is made up of fluorine and sulphur dioxide, and the second mixture of gasses emitted by the second module (2b) is made up of air and organic metal elements to improve cohesion between substrate and thin layers. A device for the realization of a process as claimed in any of the preceding claims 2 to 6, characterized by the fact that it comprises a furnace for melting raw materials, a bath of molten tin (1) for forming a glass sheet strip (V), an annealing lehr (3) provided with rollers for conveying said glass sheet strip and a fluid bed device (2) between said bath of molten tin and said roller annealing lehr for supporting and pre-cooling said glass sheet strip (V) with mixtures of gasses, characterized in that fluid bed device comprises, in the direction of travel (A) of the glass sheet strip, a first module (2a) with a length not exceeding 500 mm and a second module (2b) having a length not exceeding 1000 mm, both said modules comprising groups of blowing nozzles (20) forming the fluid bed and arranged above (S) and below (I) said glass strip (V) and inclined with respect to the horizontal by an angle of between 1.2° and 5°, and means to provide the first and second module with different mixtures of gasses. A device according to claim 7, characterized by the fact that it comprises adjacent, separate blowing tanks (5) of different widths arranged transversally to the direction of travel (A) of the glass strip (V), the tanks arranged at the edge of the strip having a smaller width than those in the center. A device according to claim 7 or 8, characterized by the fact that each blowing nozzle (20) has a slot (22) for emission of the gasses in the fluid bed, said slot being inclined in the direction of travel (A) of the glass in order to eliminate turbulence in the gasses of the fluid bed and improve the conditions for deposition of thin layers. A device according to claim 9, in which each blowing nozzle (20) is provided with a coat of refractory material (21) having the same shape as the nozzle (20) and free to rotate about a hinge (23) fixed to said nozzle, so that the inclination of said coat can be adjusted with respect to the horizontal plane.

Inventors: BOATTINI PIER PAOLO, MARIQUE JEAN CLAUDE, BOATTINI, PIER PAOLO, MARIQUE, JEAN CLAUDE
Applicants: SIV SOC ITALIANA VETRO, SOCIETA' ITALIANA VETRO - SIV - S.P.A.
Application year: 1994
Main classification: C03B18
IPC codes: C03B18, C03C23, C03B35, C03C17, C03B25